Method for reducing flue gas carbon dioxide emissions

ABSTRACT

Plants, devices, and methods are presented which economically and effectively reduce carbon dioxide (CO 2 ) emissions from flue gases by converting heat derived from one or more sources of flue gas to drive a heat engine, which generates power, energy, and/or work that is utilized by a CO 2  capture unit coupled to the stream of flue gas. CO 2  captured from the flue gas stream may be sequestered and/or utilized for commercial purposes.

FIELD OF THE INVENTION

The field of the invention is capture of CO₂ from plant emissions or waste gases. In particular this invention relates to the capture of CO₂ from flue gases, especially those produced in chemical or petrochemical plants, power plants, and refineries.

BACKGROUND

Flue gases are waste gases typically produced as exhaust from turbines, furnaces, boilers, ovens, steam generators, and similar installations, and are often produced in large quantities by power plants and chemical, petrochemical, and refinery installations. As a combustion product, the composition of flue gas is dependent on the fuel source, however, it will usually comprise nitrogen and unconsumed oxygen from the combustion air, carbon dioxide (CO₂), carbon monoxide (CO), trace components like argon, and water vapor. It may further contain volatile organic carbon compounds, nitrogen oxides, and sulfur oxides. Release of many of these compounds (CO₂ in particular) to the atmosphere has become a matter of public concern and is increasingly regulated through the implementation of fines, carbon taxes, and carbon credits.

A variety of technologies have been implemented to capture CO₂ from flue gases. These include capture by amine-based solvents, adsorption, membrane separation, and reaction with metal oxides to form carbonates. For example, the ECONAMINE FG+™ process from Fluor Corporation permits the recovery of up to 95% of the CO₂ from flue gas streams, and can produce a 99.95% pure CO₂ product (Chapel, et al). Once captured, such CO₂ may be sequestered to prevent its release to the atmosphere, or may be utilized in commercial operations such as enhanced oil recovery processes. These and all other extrinsic materials discussed herein are incorporated by reference in their entirety. Where a definition or use of a term in an incorporated reference is inconsistent or contrary to the definition of that term provided herein, the definition of that term provided herein applies and the definition of that term in the reference does not apply.

A significant barrier to removal of CO₂ from flue gases is the expense associated with the removal process. These costs can be very substantial, both in terms of equipment and operations costs and in reduction of plant efficiency. It is estimated that CO₂ capture from emissions generated by a coal-fired power plant increase overnight power generation costs by about 75% when amine-containing solvent based methods are used (Finkenrath). Even though such methods have the advantages of recycling key materials and capture and subsequent use of CO₂ at high efficiency, power requirements (and hence the energy costs) for such processes remain significant. In addition, such costs increase with the scale of the CO₂ capture operation.

Thus, there is still a need for methods and devices that reduce the CO₂ content of flue gases in an efficient and cost effective manner. Such methods and devices should, ideally, be scalable to the size of the operation, such that efficiencies and cost reductions continue to be realized as the scale of the CO₂ capture operation increases.

SUMMARY OF THE INVENTION

The inventive subject matter provides apparatus, systems, and methods in which one may economically and effectively reduce CO₂ emissions from flue gases from multiple sources by combination of the flue gases and utilization of heat from the combined flue gas stream to deliver power to a CO₂ capture unit.

One embodiment of the inventive concept is a method of reducing CO₂ emissions from flue gases by collecting hot flue gas streams from one or more flue gas sources. Typical flue gas sources include a reformer furnace, a gas turbine, a water heater, a steam generator, a boiler, and a (liquefied) natural gas heater. In preferred embodiments of the inventive concept, flue gases from two or more flue gas sources may be collected and combined to form a combined hot flue gas stream. Such a hot flue gas stream may have a temperature of about 50° C. to 250° C., and may contain CO₂ at concentration of from about 10% and 20%. Heat may recovered from all or part of such a hot flue gas stream, resulting the generation of a cooled flue gas stream that is suitable for CO₂ capture. All or part of this recovered heat may be used to drive a heat engine, which can serve to generate power. Suitable heat engines include a Rankine cycle engine, an organic Rankine cycle engine, a regenerative cycle engine, a Carnot cycle engine, a Stirling cycle engine, and a thermoelectric converter. Some or all of this power may be utilized in the recovery of CO₂ from the cooled flue gas stream. In some embodiments of the inventive concept CO₂ may be recovered from a cooled flue gas stream using an amine-based solvent. This process may generate a CO₂ stream, at least some of which can be sequestered to reduce CO₂ emissions or in any commercial application.

Another embodiment of the inventive concept is a flue gas treatment unit for capturing CO₂ from flue gas. The treatment unit includes a duct or functionally similar structure that connects a plurality of flue gas sources and a CO₂ capture unit, permitting transport of flue gas to the CO₂ capture unit. Sources of flue gas include a reformer furnace, a gas turbine, a water heater, a steam generator, a boiler, and a (liquefied) natural gas heater. Different types of flue gas sources may be connected to such a duct. A heat recovery unit is placed in thermal communication with the duct (for example, by a heat exchanger) between the plurality of flue gas sources and the CO₂ capture unit, where it may be used to extract heat. This heat may be transferred to a heat engine, which may be operatively coupled to the heat recovery unit. This heat engine may then be used to supply power to the CO₂ capture unit. Suitable heat engines include a Rankine cycle engine, an organic Rankine cycle engine, a regenerative cycle engine, a Carnot cycle engine, a Stirling cycle engine, and a thermoelectric converter. In some embodiments of the inventive concept the heat engine may include a boiler that is in thermal communication with the heat recovery unit. In other embodiments of the inventive concept the heat engine may include a turbine that is in fluid communication with such a boiler, and that is coupled to a generator or compressor. Such a generator or compressor may, in turn, be in electrical communication with the CO₂ capture unit.

Various objects, features, aspects and advantages of the inventive subject matter will become more apparent from the following detailed description of preferred embodiments, along with the accompanying drawing figures in which like numerals represent like components.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic of one exemplary embodiment of the inventive concept. A plurality of sources provide flue gas that is directed to a common duct. Heat is extracted from the flue gas, and is utilized in a heat engine that provides power used to recover CO₂ from the cooled flue gas.

FIG. 2 is a schematic of another exemplary embodiment of the inventive concept. Different sources provide flue gas that is directed to a common duct. Heat is extracted from the flue gas, and is utilized in a heat engine that provides power used to recover CO₂ from the cooled flue gas.

DETAILED DESCRIPTION

It should be noted that while the following description is drawn to methods and devices for efficiently recovering CO₂ from flue gas, various alternative configurations are also deemed suitable and may be employed to treat any suitable source of CO₂ containing gas streams, such as streams from combustion processes in the oil and gas industry, cement plants, lime kiln exhausts, engine exhausts, fermentation processes, hydrogen production plants, ammonia production plants, processing of phosphates, and so on. One should appreciate that compounds other than CO₂ may be recovered, including (but not limited to) CO, ammonia, nitrogen oxides, sulfur oxides, volatile organic carbon compounds, and chlorofluorocarbons, from gas streams containing such compounds.

One should appreciate that the disclosed techniques provide many advantageous technical effects including providing energy efficient recovery of CO₂ from flue gas, permitting CO₂ to be sequestered and thereby preventing it from being released into the atmosphere. In addition the invention advantageously supplies more power for this operation as the amount of flue gas to be processed increases, providing a synergistic effect. Such power recovery is particularly desirable where multiple flue gas streams from relatively moderate sources are combined and where CO₂ and/or heat recovery from the individual streams would not be economically or technically attractive.

The following discussion provides many example embodiments of the inventive subject matter. Although each embodiment represents a single combination of inventive elements, the inventive subject matter is considered to include all possible combinations of the disclosed elements. Thus if one embodiment comprises elements A, B, and C, and a second embodiment comprises elements B and D, then the inventive subject matter is also considered to include other remaining combinations of A, B, C, or D, even if not explicitly disclosed.

The inventive subject matter provides apparatus, systems, and methods in which one may economically and effectively reduce CO₂ emissions from flue gases (and especially from a flue gas stream that is formed from a plurality of smaller streams of distinct flue gas sources within a plant) by converting heat derived from the flue gas stream to power or energy that is utilized by a CO₂ capture unit coupled to the flue gas stream. Various objects, features, aspects and advantages of the inventive subject matter will become more apparent from the following detailed description of preferred embodiments, along with the accompanying drawing figures in which like numerals represent like components

An embodiment of the inventive concept is shown in FIG. 1. One or more sources of flue gas 100, 110 supply streams of hot flue gas to a collecting duct 120 to form a combined hot flue gas stream 125. Such hot flue gas streams, which may (for example) result from combustion processes utilized in the coal, gas, and/or petroleum industries (such as, for example, from a reformer furnace, a gas turbine, a water heater, a steam generator, a boiler, and/or a (liquefied) natural gas heater) are generally available at elevated temperatures and low to moderate pressures. In some embodiments of the inventive concept the (combined) hot flue gas stream may have a temperature ranging from about 40° C. to about 450° C. and/or pressure ranging from about −100 mbarg to about +300 mbarg. In a preferred embodiment of the inventive concept the (combined) hot flue gas stream has a temperature ranging from about 50° C. to about 250° C. and/or a pressure ranging from about −50 mbarg to about +200 mbarg. In some embodiments of the inventive concept the CO₂ content of such flue gas can range from about 1% to about 60%. In other embodiments of the inventive concept the CO₂ content of such flue gas can range from about 5% to about 50%. In a preferred embodiment of the inventive concept the CO₂ content of such flue gas can range from about 10% to about 20%. Unless the context dictates the contrary, all ranges set forth herein should be interpreted as being inclusive of their endpoints, and open-ended ranges should be interpreted to include commercially practical values. Similarly, all lists of values should be considered as inclusive of intermediate values unless the context indicates the contrary.

It should be recognized that sources of flue gas may operate at different pressures and flow rates, and that embodiments of the inventive concept may incorporate suitable control devices to permit safe and effective blending of these. For example, a control device may be incorporated into a flue gas source that prevents transfer of flue gas to the duct 120 when the pressure of the flue gas is below a specified value. Such a condition may occur when a flue gas source 100, 110 is offline or is in a specific portion of a power cycle. In such an embodiment of the inventive concept a control device may include a monitoring device that characterizes the flue gas. Characteristics of the flue gas that may be monitored for control purposes may include (but are not limited to) pressure, temperature, CO₂ content, and/or a combination of these. This combining of flue gas streams from multiple sources advantageously permits the generation of sufficient volume of hot flue gas to make heat energy recovery from this material commercially viable.

In some embodiments of the inventive concept heat may be recovered from the combined hot flue gas 125 by a heat recovery unit 130. In some embodiments of the inventive concept the heat recovery unit is in thermal communication with the duct 120. Such a heat recovery unit 130 may, for example, include a heat exchanger that is in thermal contact with the duct 120. Suitable heat exchangers include, but are not limited to, a counterflow heat exchanger (of vertical, horizontal, or cellular design), a heat pipe, a liquid-coupled heat exchanger or run-around coil, a rotary enthalpy wheel, or a combination of these. Removal of heat from combined hot flue gas 125 generates a stream of cooled flue gas 135 that may be directed to a CO₂ capture unit 140 that is in fluid communication with the duct 120. The reduced temperature of the cooled flue gas 135 is advantageous to many CO₂ capture processes. In some embodiments of the inventive concept the temperature of the cooled flue gas 135 may be about 10° C. lower than that of the hot flue gas 125. In other of the inventive concept the temperature of the cooled flue gas 135 may be about 20° C. lower than that of the hot flue gas 125. In still other embodiments of the inventive concept the temperature of the cooled flue gas 135 may be about 30° C. or more lower than that of the hot flue gas 125. The CO₂ capture unit 140 may employ any suitable CO₂ capture technology. Suitable technologies include, but are not limited to, extraction with (e.g., amine-containing) solvents, membrane separation, cryogenic treatment, pressure swing adsorption, or any reasonable combination of these. In a preferred embodiment of the inventive concept the CO₂ capture unit utilizes an ECONAMINE FG+™ process (Fluor Corporation, Irving, Tex. USA, 75039). The CO₂ capture unit 140 may, in turn, produce a stream of CO₂-depleted flue gas 180 and a stream of CO₂ 190 recovered from the cooled flue gas 135. In some embodiments of the inventive concept the recovered CO₂ 190 is sequestered in order to prevent its release into the atmosphere. Suitable CO₂ sequestration techniques include, but are not limited to, subterranean injection into depleted fossil fuel reserves, injection into basalt, and/or reaction with metal oxides (such as magnesium of calcium oxide) to form carbonate minerals. Such carbonate minerals may advantageously be utilized as building materials. Alternatively, CO₂ recovered by this process may be utilized as a carbon source for the cultivation of algae and/or in the synthesis of hydrocarbons and oxygen-containing organic compounds (such as methanol) or as growth enhancer in greenhouses.

Such CO₂ capture methods require energy or power to operate. In an embodiment of the inventive concept all or part of this energy or power may be supplied by a heat engine 160 that utilizes at least a portion of the heat 150 supplied by a heat recovery unit 130 and derived from a hot flue gas stream 125 to generate power 170 that is transmitted to a CO₂ capture unit 140. For example, CO₂ may be compressed following release from an amine-containing solvent using energy derived from heat recovered from the hot flue gas stream 125. Suitable heat engines include, but are not limited to a Rankine cycle engine, an organic Rankine cycle engine, a regenerative cycle engine, a Carnot cycle engine, a Stirling cycle engine, a thermoelectric device, or a combination of these. In some embodiments of the inventive concept the heat engine 160 may drive a generator that supplies electric power (or in the case of a thermoelectric device, generates) to the CO₂ capture unit 140. In such an embodiment a portion of the electric power thus generated may be utilized to run other plant operations, such as lighting or climate control. In other embodiments of the inventive concept, the heat engine 160 may directly or indirectly (for example, by supplying current to an electric motor) drive a compressor that supplies pressure to the CO₂ capture unit 140. In yet another embodiment of the inventive concept the heat engine 160 may supply mechanical work to the CO₂ capture unit 140, for example driving one or more pumps or compressors. In some embodiments of the inventive concept, two or more heat engines 160 may utilize heat from a heat recovery unit 130, and may in turn supply different forms of power to the CO₂ capture unit 140. In other embodiments of the inventive concept the heat engine 160 may also supply power to other parts of a plant or installation. Most preferably, however, heat from the heat recovery unit may be used in the process of regeneration of the solvent and/or in the generation of electrical or mechanical energy to reduce CO₂ compression requirements.

FIG. 2 schematically illustrates another embodiment of the inventive concept. In such an embodiment flue gases are collected from two or more sources 200, 205, 210, and 215. These sources may be of different types; for example, source 200 and source 205 may both produce suitable flue gases but by different means. Suitable sources include, but are not limited to devices utilized for combustion processes utilized in the coal, gas, and/or petroleum industries such as, for example, a reformer furnace, a gas turbine, a water heater, a steam generator, a boiler, and/or a (liquefied) natural gas heater. Flue gas sources are collected in a duct 220. In some embodiments of the inventive concept, flue gas may be collected in a secondary duct 225 that joins another duct 220. In some embodiments of the inventive concept the combined flue gases collected from these sources may have a temperature ranging from about 40° C. to about 450° C. and/or pressure ranging from about −100 mbarg to about +300 mbarg. In a preferred embodiment of the inventive concept the hot flue gas stream has a temperature ranging from about 50° C. to about 250° C. and/or a pressure ranging from about −50 mbarg to about +200 mbarg. In some embodiments of the inventive concept the CO₂ content of such flue gas can range from about 1% to about 60%. In other embodiments of the inventive concept the CO₂ content of such flue gas can range from about 5% to about 50%. In a preferred embodiment of the inventive concept the CO₂ content of such flue gas can range from about 10% to about 20%.

It should be recognized that utilization of diversified sources of flue gas may result in flue gas being supplied at different pressures and flow rates, and that embodiments of the inventive concept may incorporate suitable control devices to permit safe and effective blending of these. For example, a control device may be incorporated into a flue gas source 200, 205, 210, and/or 215 that prevents transfer of flue gas to a duct 220, 225 when the pressure of the flue gas is below a specified value. Similarly, a control device may be used to control the flow of pooled gases between ducts (not shown). Such a condition may occur when a flue gas source 200, 205, 210, and/or 215 is offline, intermittently operating, or in a specific portion of a power cycle. In such an embodiment a control device may include a monitoring device that measures one or more characteristics of the flue gas. Characteristics of the flue gas that may be monitored for control purposes may include (but are not limited to) pressure, temperature, CO₂ content, and/or a combination of these. This combining of flue gas streams from multiple and diverse sources advantageously permits the generation of sufficient volume of hot flue gas to make heat energy recovery from this material commercially viable.

In order to support recovery of CO₂ from the pooled flue gases, the duct 220 may be in fluid communication with a CO₂ capture unit 235. In an embodiment of the inventive process, heat is extracted from the flue gas by a heat recovery unit 230, that may be placed in thermal contact with the duct 220 prior to its connection to the CO₂ capture unit 235. Removal of heat from the pooled flue gases reduces its temperature, which is advantageous to many CO₂ removal processes. In some embodiments of the inventive concept the temperature of the flue gas may be reduced by about 10° C. following passage through the heat recovery unit 230. In another embodiment of the inventive concept the temperature of the flue gas may reduced by about 20° C. following passage through the heat recovery unit 230. In still other embodiments of the inventive concept the temperature of the flue gas may be reduced by about 30° C. or more following passage through the heat recovery unit 230. The CO₂ capture unit 235 may employ any suitable CO₂ capture technology. Suitable technologies include, but are not limited to, extraction with amine-containing solvents, cryogenic treatment, pressure swing adsorption, or a combination of these. For example, CO₂ may be compressed following release from an amine-containing solvent using energy derived from heat recovered from the flue gases contained in the duct 220. In a preferred embodiment of the inventive concept the CO₂ capture unit 235 utilizes an ECONAMINE FG+™ process (Fluor Corporation, Irving, Tex. USA, 75039). The CO₂ capture unit 235 may, in turn, produce a stream of CO₂-depleted flue gas 290 and a stream of CO₂ 295. Such a stream of CO₂ 295 may be sequestered or otherwise utilized to reduce release of CO₂ emissions to the atmosphere.

The heat that is recovered from the flue gases in the duct 220 by the heat recovery unit 230 may advantageously be utilized as a source of energy or work in the CO₂ capture unit 235. As noted above, this advantageously provides more energy that may be utilized for CO₂ capture as the volume flue gases to be processed increases, providing a synergistic effect. In some embodiments of the inventive concept the heat recovery unit 230 is in thermal communication with the duct 220. Such a heat recovery unit 230 may, for example, include a heat exchanger that is in thermal contact with the duct 220. Suitable heat exchangers include, but are not limited to, a counterflow heat exchanger (of vertical, horizontal, or cellular design), a heat pipe, a liquid-coupled heat exchanger or run-around coil, a rotary enthalpy wheel, or a combination of these. Removal of heat from flue gas in the duct 220 generates a stream of cooled flue gas that may be directed to a CO₂ capture unit 235 that is in fluid communication with the duct 220. Such heat 240 removed from the flue gas in the duct 220 may be transformed into power, energy, and/or work by a heat engine 245. Suitable heat engines include, but are not limited to a Rankine cycle engine, an organic Rankine cycle engine, a regenerative cycle engine, a Carnot cycle engine, a Stirling cycle engine, a thermoelectric device, or a combination of these.

In FIG. 2, an exemplary heat engine 245 is shown with a boiler 250 that utilizes heat recovered by the heat recovery unit 230 to heat a working fluid, thereby causing it to expand. The resulting increase in pressure is transmitted via a high pressure line 265 to a turbine 255, causing it to spin. Such a turbine 255 may be utilized to drive a generator, compressor, mechanical linkage, or other device (not shown) to produce transmissible energy, power, or work 285, which may be used to drive processes utilized in the CO₂ capture unit 235. Following expansion in the turbine 255, the fluid may be transferred via a low pressure line 270 to a condenser 260. The condensed fluid may then be returned to the boiler 250 via a condensate line 280. It should be recognized that in some embodiments of the inventive concept heat 240 from the heat recovery unit may be distributed to two or more heat engines. In such an embodiment the heat engines may supply energy, power, or work in different forms.

Other approaches to improve efficiencies through the use of flue gases have been disclosed. For example, U.S. Pat. No. 5,114,682 (to Goelzer) discloses burning a combustible byproduct of petrochemical catalyst regeneration and combining the resulting hot, pressurized gases with flue gas to drive a turbine. Similarly, E.P. Patent Application No. 2,022,837 A1 (to Zhu) describes burning a “dry gas” byproduct of a fluid catalytic cracking process to heat flue gas, subsequently utilizing the expanding flue gas to drive a turbine. In both of these, however, the flue gas is utilized as a mere working fluid, which is heated by a secondary source in order to perform work. Neither addresses the issue of reducing the CO₂ content of the flue gas that is released into the atmosphere; in fact, the process described in U.S. Pat. No. 5,114,682 would actually increase the CO₂ content of the released gas. In addition, since heat that is already present in the flue gas is not being directly harnessed to reduce CO₂ emissions, it is not evident that processes such as these are readily scalable as the amount of flue gas to be processed increases.

In alternative embodiments of the inventive concept, a portion of the heat 240 recovered by the heat recovery unit 230 may be utilized without conversion to energy, power, or work. For example, a portion of the heat recovered from flue gases in the duct 220 may be transferred to the CO₂ capture unit 235 as heat, where it may be used to heat a reboiler (not shown) used to release CO₂ captured from the flue gas by an amine-containing solvent. Similarly, a portion of the heat 240 recovered by the thermal recovery unit 230 may be utilized to pre-heat fuel and/or oxygen sources for one or more of the flue gas sources 200, 205, 210, and 215.

In still other embodiments of the inventive concept, heat recovered from fluidic waste streams containing undesirable compounds other than CO₂ may be used to provide power for capture of such undesirable compounds from the waste fluidic stream. Examples of such compounds include, but are not limited to CO, ammonia, nitrogen oxides, sulfur oxides, volatile organic carbon compounds, and chlorofluorocarbons. Once recovered, such undesirable compounds may be reutilized, sequestered, or repurposed in order to reduce their release into the environment. In some embodiments of the inventive concept, one or more capture units configured to perform capture processes for different compounds (or families of compounds) may be placed in fluid communication with a duct that is, in turn, in communication with one or more sources of a stream of flue gas containing such compounds. Heat recovered from such a flue gas stream utilizing a heat recovery system may be used by a heat engine to provide energy, power, or work that may, in turn, be utilized to drive one or more of these capture units. In some embodiments of the inventive concept a heat recovery unit may be placed downstream from one or more capture units. In other embodiments of the inventive concept one or more heat recovery units may be positioned between capture units, allowing temperature of the fluid stream to be modulated as appropriate for individual capture units. Still further, it should be appreciated that upon cooling of the combined hot flue gas, water may be condensed and recovered in significant quantities, which may be employed as boiler feed water, make-up water (e.g., for amine solvent), or other purpose within the plant.

As used herein, and unless the context dictates otherwise, the term “coupled to” is intended to include both direct coupling (in which two elements that are coupled to each other contact each other) and indirect coupling (in which at least one additional element is located between the two elements). Therefore, the terms “coupled to” and “coupled with” are used synonymously.

It should be apparent to those skilled in the art that many more modifications besides those already described are possible without departing from the inventive concepts herein. The inventive subject matter, therefore, is not to be restricted except in the scope of the appended claims. Moreover, in interpreting both the specification and the claims, all terms should be interpreted in the broadest possible manner consistent with the context. In particular, the terms “comprises” and “comprising” should be interpreted as referring to elements, components, or steps in a non-exclusive manner, indicating that the referenced elements, components, or steps may be present, or utilized, or combined with other elements, components, or steps that are not expressly referenced. Where the specification claims refers to at least one of something selected from the group consisting of A, B, C . . . and N, the text should be interpreted as requiring only one element from the group, not A plus N, or B plus N, etc. 

What is claimed is:
 1. A method of reducing CO₂ emissions from flue gases comprising; collecting and combining a plurality of hot flue gas streams from a plurality of respective flue gas sources to thereby form a combined hot flue gas stream; recovering at least a portion of heat from the combined hot flue gas stream to generate a cooled flue gas stream; utilizing at least a portion of the recovered heat to drive a heat engine to generate power, wherein at least a portion of the generated power is used to remove CO₂ from the cooled flue gas stream.
 2. The method of claim 1, wherein the plurality of flue gas sources includes a device selected from the group consisting of a reformer furnace, a gas turbine, a water heater, a steam generator, a boiler, and a liquefied natural gas heater.
 3. The method of claim 1, wherein the combined hot flue gas stream has a temperature between about 50° C. and about 250° C.
 4. The method of claim 1, wherein the combined hot flue gas stream has a CO₂ concentration of between about 10% and 20%.
 5. The method of claim 1, wherein the heat engine is selected from the group consisting of a Rankine cycle engine, an organic Rankine cycle engine, a regenerative cycle engine, a Carnot cycle engine, a Stirling cycle engine, and a thermoelectric converter.
 6. The method of claim 1, wherein CO₂ is captured using an amine-based solvent.
 7. The method of claim 1, wherein capture of CO₂ from the cooled flue gas stream generates a CO₂ waste stream.
 8. The method of claim 7, wherein at least a portion of the CO₂ waste stream is sequestered.
 9. A flue gas treatment unit for capturing CO₂ from flue gas, comprising; a duct that is in fluid communication with a plurality of flue gas sources and a CO₂ capture unit; wherein the duct is configured to form a combined hot flue gas stream from a plurality of hot flue gas streams of the plurality of flue gas sources, respectively; a heat recovery unit that is in thermal communication with the duct to recover heat from the combined hot flue gas stream and is interposed between the plurality of flue gas sources and the CO₂ capture unit; and a heat engine operatively coupled to the heat recovery unit and configured to supply power to the CO₂ capture unit.
 10. The flue gas treatment unit of claim 9, wherein the plurality of flue gas sources includes a device selected from the group consisting of a reformer furnace, a gas turbine, a water heater, a steam generator, a reboiler, and a liquefied natural gas heater.
 11. The flue gas treatment unit of claim 9, wherein the plurality of flue gas sources comprise different types of flue gas sources.
 12. The flue gas treatment unit of claim 9, wherein the heat recovery unit is a heat exchanger.
 13. The flue gas treatment unit of claim 9, wherein the heat engine is selected from the group consisting of a Rankine cycle engine, an organic Rankine cycle engine, a regenerative cycle engine, a Carnot cycle engine, a Stirling cycle engine, and a thermoelectric converter.
 14. The flue gas treatment unit of claim 9, wherein the heat engine comprises a boiler that is in thermal communication with the heat recovery unit.
 15. The flue gas treatment unit of claim 14, further comprising a turbine that is in fluid communication with the boiler and is operatively coupled to a generator or compressor.
 16. The flue gas treatment unit of claim 15, wherein the generator or compressor is in electrical communication with the CO₂ capture unit. 